What changes the rate of evaporation — questions

1. identify / explain

A pupil wants to find out whether moving air speeds up evaporation from towel B. She does one run in still air and one with the fan on. Which things must she keep the same in both runs? Choose all that apply, then explain why.

Tick every correct statement.

• The mass of water on towel B at the start (3.00 g)
• The starting temperature of the water in the tubes (70 °C)
• How much of the towel is open to the air
• The time of each run (5 minutes)
• Whether the fan is on
• The reading on balance B at the end

Why must these stay the same?

Nudge (only if you are stuck): Only one thing should change between the runs: the one being tested. The reading at the end is a result, not something she sets.

2. calculate / explain

With the fan on, balance B read 81.23 g at the start and 80.31 g after 5 minutes. In still air, 0.64 g of water evaporated in the same time. Calculate (a) the mass of water that evaporated with the fan on and (b) how much more water evaporated with the fan on than in still air. Then explain the difference.

(a) Water evaporated with the fan on: ______________ unit ________

(b) How much more than in still air: ______________ unit ________

Why did more water evaporate with the fan on?

Nudge (only if you are stuck): (a) The reading at the start − the reading at the end. (b) The fan run − the still-air run.

3. complete

Complete the sentences. Evaporation is faster when the liquid is ____ (1), because more of its particles have enough energy to escape. It is faster when the surface area is ____ (2), because more particles are at the surface. It is faster when air moves over the surface, because the moving air carries the escaped particles ____ (3), so fewer of them ____ (4) to the liquid.

Gap 1

Gap 2

Gap 3

Gap 4

Nudge (only if you are stuck): Think of washing on a line: which conditions dry it fastest?

4. identify

Match each change to the wet towel to its effect on the rate of evaporation and the reason.

Write the number of the matching option.

1. Faster: the moving air carries the escaped particles away, so fewer return
2. Slower: fewer particles are at an open surface where they can escape
3. Slower: fewer particles have enough energy to escape
4. Faster: the moving air makes the particles more energetic
5. No change: the same water is on the towel

• Switching on a fan that blows over the wet towel: ______
• Covering half of the wet towel with cling film: ______
• Starting from water at 50 °C instead of 70 °C: ______

Nudge (only if you are stuck): Each change acts on one factor: air movement, surface area or temperature.

5. identify / explain

Which statement describes a difference between boiling and evaporation? Then explain your choice.

Circle one answer.

• Boiling happens only at the boiling point; evaporation happens at any temperature
• Boiling turns a liquid into a gas; evaporation turns a gas into a liquid
• Evaporation happens throughout the liquid; boiling happens only at the surface
• Evaporation needs energy supplied all the time; boiling does not

Why?

Nudge (only if you are stuck): Think of a puddle drying on a cool day, and a pan bubbling on a hob.

6. identify

Match each statement to boiling, to evaporation, or to both.

Write the number of the matching option.

1. boiling
2. evaporation
3. both
4. neither

• Happens at any temperature: ______
• Happens only from the surface of the liquid: ______
• Bubbles of vapour form throughout the liquid: ______
• Happens only at one temperature, the boiling point: ______
• A liquid turns into a gas: ______
• Needs energy supplied to the liquid all the time to keep going: ______

Nudge (only if you are stuck): Picture a puddle drying on a cool day and a pan bubbling on a hob. Which statements fit the puddle, which the pan, and which both?

7. calculate / explain

In the half-covered run, the cling film added 0.35 g, so balance B started at 81.58 g. After 5 minutes it read 81.19 g. In the run from 50 °C, with the whole towel open, balance B fell from 81.23 g to 80.88 g. Calculate the mass of water that evaporated in (a) the half-covered run and (b) the run from 50 °C. Then explain why each lost less than the 0.64 g lost in still air from 70 °C.

(a) Water evaporated, half covered: ______________ unit ________

(b) Water evaporated, from 50 °C: ______________ unit ________

Why did each run lose less than 0.64 g?

Nudge (only if you are stuck): For each run, the reading at the start − the reading at the end. The cling film stays on the balance the whole time.

8. explain

With the fan on, tube B fell by 36.0 °C in 5 minutes, instead of 25.5 °C in still air. A pupil says: “The fan made tube B fall 10.5 °C further, so that shows how much faster the water evaporated.” Tube A fell by 19.0 °C with the fan on and by 11.0 °C in still air. Explain why the falls in temperature cannot, on their own, show how much faster the water evaporated, and say which reading should be used instead.

Nudge (only if you are stuck): Tube A had no water on its towel. What did the fan do to it?

9. identify / explain

Four identical wet towels are hung up to dry. Which one dries fastest? Then explain your choice.

Circle one answer.

• Folded in a basket in a cold, still shed
• Spread out on a line outdoors, on a cold, still day
• Spread out on a line outdoors, on a warm, windy day
• Folded over a line outdoors, on a warm, windy day

Why?

Nudge (only if you are stuck): Check all three factors for each towel: temperature, surface area and air movement.

10. explain / describe

A pan of water on a low heat is at 60 °C. Mist rises above it, but no bubbles form in it. A pupil says the water is boiling. Explain why the water is evaporating, not boiling. Then describe one more difference between boiling and evaporation.

Nudge (only if you are stuck): Use the two clues in the question: the temperature and the bubbles.

11. describe / explain / suggest

A bowl of soup at 80 °C is too hot to eat. Mist rises from its surface, but it is not boiling. (a) Describe two differences between what is happening at the soup’s surface and boiling. (b) Blowing across the soup cools it faster. Explain why, in terms of particles. (c) Pouring the soup onto a wide, flat plate also cools it faster. Explain why. (d) A cook puts a lid on the bowl to keep the soup hot. Suggest how the lid slows the cooling. (e) As the soup cools from 80 °C to 40 °C, the mist above it fades, and the soup evaporates more slowly. Explain why it evaporates more slowly, in terms of particles.

(a) Two differences from boiling

(b) Blowing across it

(c) A wide plate

(d) A lid

(e) As the soup cools

Nudge (only if you are stuck): Use the boiling and evaporation table for (a), and the three factors from the experiment for (b) to (e).